1. Field of the Invention
The invention in general relates to medical prostheses for implantation in a body, and more particularly to prostheses which are fixed within the body by means of bone ingrowth into the prostheses.
2. Description of the Prior Art
Medical devices such as bone plates, nails, pins, screws, and joint prostheses have been commonly implanted into the skeletal structure of humans and animals for many years to join the parts of a fractured bone or to replace missing or damaged skeletal parts. Often it is intended that these parts become a permanent part of the body. In such cases it is important that the parts be strongly and permanently fixed to the skeletal bone structure.
It has been known for more than 10 years that strong permanent fixation can be obtained by the use of a porous surface on the whole or a portion of the part, provided that the mean pore size exceeds about 50 micrometers. However, the older methods of bone fixation, primarily fixation by friction fit or with methyl methacrylate bone cement, still are the most predominately used methods of fixation, in spite of the fact that the loosening of friction fitted and cement bonded prostheses parts over time remains a significant medical problem. This is because in order to obtain good fixation by means of bone ingrowth, the patient must refrain from applying force or loading on the skeletal implant until the bone growth occurs, whereas in the case of friction fit or cement bonding skeletal loading can take place almost immediately.
In this disclosure, the words bioabsorbable and resorable mean that the substance to which the term is applied is broken down, absorbed, or otherwise removed by the host body chemistry in amounts sufficient to alter the physical structure of the portion of the device which is bioabsorbable within a time period less than the period it takes for bones to fully mend. It is understood that all materials, even steel, are slightly absorbed by the body chemistry, however, such slight absorbtions, which do not alter the physical structure of the bioabsorbable device over the period of bone healing, are not included in the terms bioabsorbable and resorbable. it is also understood that although a material is bioabsorbable, some small amount of it may remain in the body for longer periods.
One method shown to encourage earlier bone ingrowth into porous surfaces of implants is the coating of porous metal fibers with hydroxyapatite by dipping the fiber into a water slurry of hydroxyapatite and drying. Hydroxyapatite is not generally considered to be resorbable in the human body. It was found that the hydroxyapatite encourages more rapid ingrowth of bone into the porous metal surfaces for a time period of up to four weeks after implant, but that the effect was short term, in that the amount of bony tissue within the pores declined after the four week period, P. Ducheyne, et al. "Effect of Hydroxyapatite Impregnation on Skeletal Bonding of Porous Coated Implants" Journal of Biomedical Materials Research, Vol. 14, 225-237 (1980). In addition to the fact that the increased ingrowth was not permanent, it has been found that the hydroxyapatite encourages fibrous rather than bony tissue growth at the fixation site over long time periods, which can result in loosening of the prostheses. Further, the hydroxyapapite coating in the Ducheyne et al. article is relatively fragile and can be easily broken under normal handling for commercial products, and thus it does not lend itself to widespread commercial use.
The following United States patents relate to the aspects of the present invention as indicated. U.S. Pat. No. 3,605,123 issued to H. Hahn discloses the plasma spraying of a metal porous surface onto a prosthesis. U.S. Pat. Nos. 3,892,648 and 3,892,649 issued to David C. Phillips et al. disclose the electrodeposition of bone and collagen on implants or into a plastic mesh on implants to stimulate bone attachment to the implant.
U.S. Pat. No. 3,919,723 discloses the embedding of calcium and phosphate atoms in the surface of a ceramic implant by heating the implant and embedding it in a melt of calcium phosphate material. It is specifically indicated that temperatures should not be used which decompose one of the materials. calcium phosphate material decomposes at higher temperatures. This decomposition at higher temperatures is characteristic of calcium phosphate materials. See E. Hayek and H. Newesely, Inorganic Synthesis 7 (1963) 63.
U.S. Pat. No. 4,202,055 issued to Reiner et al. discloses the combining of a bioabsorbable, bioactive calcium phosphate with a polymer on the surface of a prosthesis, to create bone ingrowth into the polymer. U.S. Pat. Nos. 4,365,357 and 4,373,217 issued to Draenert disclose the combining of the absorbable tricalcium phosphate material with bone cement to create bone growth into the cement. Each of the above three patents involve the incorporation of the absorbable material into the material out of which the surface is composed, and contemplate that the porous surface is created by absorption of the absorbable material thereby leaving pores in the surface in the position of the vacancy created by the absorption of the absorbable material. None of these patents suggest the coating of an already porous surface with an absorbable material in order to enhance bone growth.
U.S. Pat. No. 4,338,926 issued to Kummer et al. discloses the addition of a bioabsorbable layer 0.1 to 1 mm thick on an implant surface; the intention is to create loosening of the implant as the material is absorbed. The prosthesis on which the absorbable layer is placed is specifically non-porous, since bone ingrowth is to be discouraged.